1. Field of the Invention
The present invention relates to a radiation image detector that records a radiation image by generating charges by irradiation with radiation and by storing (accumulating) the charges.
2. Description of the Related Art
Conventionally, in the medical field or the like, various types of radiation image detectors that record radiation images (radiographic images) of subjects by irradiation with radiation that has passed through the subjects have been proposed and used.
One of the examples of the radiation image detectors is a radiation image detector using amorphous selenium, which generates charges by irradiation with radiation. As such a radiation image detector, a light-readout-type radiation image detector and an electric-readout-type radiation image detector have been proposed.
As the radiation image detector of the light-readout-type, a radiation image detector as illustrated in FIG. 20 has been proposed, for example. In the radiation image detector, a first electrode layer 101, a photoconductive layer 102 for recording, a charge transfer layer 103, a photoconductive layer 104 for readout, a second electrode layer including a transparent linear electrode 106 and a light-shield linear electrode 107 are superposed one on another in this order. The first electrode layer 101 transmits radiation that carries a radiation image, and the photoconductive layer 102 for recording generates charges by irradiation with the radiation that has passed through the first electrode layer 1. The charge transfer layer 103 acts as an insulator for charges having one of the polarities of the charges generated in the photoconductive layer 102 for recording and acts as a conductor for charges having the other polarity. The photoconductive layer 104 for readout generates charges by irradiation with readout light. The transparent linear electrode 106 transmits the readout light and the light-shield linear electrode 107 blocks the readout light.
When a radiation image is recorded in a light-readout-type radiation image detector, as described above, first, a negative voltage is applied to the first electrode layer 101 of the radiation image detector by a high-voltage power source. Then, while the negative voltage is applied, radiation that has been transmitted through a subject, and which carries a radiation image of the subject, is output to the radiation image detector from the first electrode layer 101 side.
Then, the radiation that has been output to the radiation image detector passes through the first electrode layer 101 and irradiates the photoconductive layer 102 for recording. Then, dipoles (pairs of charges, electrons and holes) are generated in the photoconductive layer 102 for recording by irradiation with the radiation. The positive charges of the dipoles combine with negative charges charged in the first electrode 101 and disappear. The negative charges of the dipoles are accumulated, as latent image charges, in a charge storage portion 105 that is formed at the interface between the photoconductive layer 102 for recording and the charge transfer layer 103, and a radiation image is recorded (please refer to FIG. 20).
Next, while the first electrode layer 101 is set in a grounded state, readout light is output to the radiation image detector from the second electrode layer side. The readout light is transmitted through the transparent linear electrode 106 and irradiates the photoconductive layer 104 for readout. Positive charges generated in the photoconductive layer 104 for readout by irradiation with the readout light combine with the latent image charges in the charge storage portion 105. Further, an electric current that flows when negative charges combine with positive charges charged in the transparent linear electrode 106 and the light-shield linear electrode 107 is detected by a charge amplifier connected to the light-shield linear electrode 107. Accordingly, an image signal corresponding to the radiation image is read out.
When a radiation image is recorded in a radiation image detector, as described above, a negative voltage is applied to the first electrode layer 101. At this time, an electric field is concentrated in the vicinity of the edge of the first electrode layer 101. Therefore, charges are injected from the first electrode layer 101 into the photoconductive layer 102 for recording. Hence, there has been a problem that fluctuation in the density tends to occur at the edge portion of the radiation image. Further, there has been a risk of discharge breakdown by creeping discharge at the edge portion of the first electrode layer 101.
Meanwhile, as the radiation image detector of the electric-readout-type, a radiation image detector in which an upper electrode, to which a voltage is applied, a semiconductor layer and an active matrix substrate are superposed one on another has been proposed, for example. The semiconductor layer generates charges by irradiation with radiation. Further, in the active matrix substrate, a multiplicity of pixels, each including a collection electrode, a storage capacity and a TFT switch, are two-dimensionally arranged. The collection electrode collects the charges generated in the semiconductor layer. The storage capacity stores the charges collected by the collection electrode, and the TFT switch is used to read out the charges stored in the storage capacity.
When a radiation image is recorded in the radiation image detector of the electric-readout-type, as described above, first, a positive voltage is applied to the upper electrode of the radiation image detector by a voltage source. Then, while the positive voltage is applied to the upper electrode, radiation that has passed through a subject, and which carries a radiation image of the subject, is output to the radiation image detector from the upper-electrode side.
The radiation that has been output to the radiation image detector is transmitted through the upper electrode, and the semiconductor layer is irradiated with the radiation. Then, dipoles are generated in the semiconductor layer by irradiation with the radiation. Negative charges of the dipoles combine with positive charges charged in the upper electrode and disappear, and positive charges of the dipoles are collected, as latent image charges, by the collection electrode of each pixel in the active matrix substrate. Further, the positive charges are stored in each storage capacity, and the radiation image is recorded.
Then, the TFT switch of the active matrix substrate is turned on based on a control signal output from a gate driver, and the charges stored in the storage capacity are read out. The charge signals of the charges are detected by a charge amplifier. Consequently, image signals corresponding to the radiation image are read out.
However, in the radiation image detector of the electric-readout-type, when the positive voltage is applied to the upper electrode as described above, concentration of an electric field in the vicinity of the edge portion of the upper electrode occurs. Therefore, charges are injected from the upper electrode into the semiconductor layer. Hence, there has been a problem that fluctuation in the density tends to occur at the edge portion of the radiation image. Further, there has been a risk of discharge breakdown by creeping discharge at the edge portion of the upper electrode.
U.S. Pat. No. 6,885,005 proposes a radiation image detector using amorphous selenium. In the radiation image detector, an insulating material that has high withstand-voltage is formed between the edge portion of the voltage-applied electrode, to which a voltage is applied, and the semiconductor layer to prevent injection of charges and discharge breakdown as described above.
However, when the insulating material is provided between the edge portion of the voltage-applied electrode and the semiconductor layer, as in the radiation image detector disclosed in U.S. Pat. No. 6,885,005, it is impossible to form a sufficient electric field in the semiconductor layer that corresponds to the area in which the insulating material has been provided. Therefore, it is impossible to generate sufficient charges and to read out sufficient image signals. In other words, it is impossible to use the area as an image area. Consequently, there has been a problem that the image area becomes small.